The present invention relates generally to apparatus for heating steel or other metal in a semi-continuous type reheat furnace which heats the product in one or more rows and discharges the heated product for further processing.
Reheat furnaces can generally be described as batch type, continuous, and semi-continuous.
The batch type reheat furnace is represented by the solid brick hearth furnace which receives product in several pieces at substantially the same time, they are heated altogether, and each is drawn from the furnace as required by the process, the remaining pieces are stored until needed.
The continuous reheat furnace is a type of furnace where long product pieces are rolled continuously through the furnace of 300 to 400 feet long, and thus the longitudinal line of the furnace and the steel are coincident. Also, this type of furnace is so long, that there is insufficient room for the waste gases to flow inside of the furnace. As shown by Wilde, U.S. Pat. No. 3,291,465, these gases are ducted off to the side and do not present any interference to the flow of heating flames.
A semi-continuous reheat furnace may have one to three rows of furnace product present in the furnace at one time while being heated from a cold condition to a temperature of about 2200 degrees Fahrenheit. The steel temperature depends on the position as the product advances through the furnace. There are several heating zones in this type of reheat furnace followed by a soaking zone. The soak zone is especially designed to mitigate temperature differences in the product, for example, between the outer surface and the center, or between the edges and the center, with burners which provide a greater spread of radiating energy more or less evenly over all of the product. This soak zone is equipped with a firing rate capability which is considerably less than the heating zones, because the purpose of the soak zone, is that of evening out temperature differences. To the extent that there are cold skid marks on the product as it enters the soak zone, these are generally not erased and in order to do so, it is necessary to delay furnace production so that this zone may accommodate this condition.
One such type of semi-continuous reheat furnace is the five-zone furnace disclosed in the Sidwell U.S. Pat. No. 3,148,868. This patent is an attempt to improve five-zone average furnace productivity, by the judicious placement of preheated air.
A conventional way of advancing the work in the furnace is by pushing each entering piece of work into the entry end of the furnace causing it to engage and push forward the line of working pieces ahead, which are already undergoing heating and soaking. The foremost in such heating line is discharged contemporaneously with, and by the entry of a fresh workpiece.
Another way of advancing the work, is to push the workpiece onto water-cooled mechanical walking beams, which space the work, one from the other. From thence, the work will advance as it is walked through the furnace. This mechanism inside the furnace adds additional water-cooled members.
Thus, the definition of a semi-continuous furnace, because of the distance of each movement is only as far as it is necessary to discharge the foremost piece onto the mill tables. Then the process halts until another demand for another piece of steel is required.
In the process of heating, the steel rests on skid bars supported by water-cooled pipes. To conserve heat, the water pipes are insulated, the effective condition of the insulation varies during a furnace campaign.
The normal passage of steel through the furnace is subject to delays in production on one hand or to high production rates on the other because of high or low mill demand, causing the steel to rest on the skids for varying time periods.
In addition, one or more of the rows of steel inside of the furnace, may, for various reasons be delayed with respect to another row, so that it sits idly in the furnace while another row is being pushed to an excessive rate.
As may be seen, the cold skid marks form on the product as soon as the product rests on any water-cooled skid bar for any appreciable period of time.
The heating zones are each provided with a conventional control means which controls a multiplicity of burners that are designed and adjusted to fire evenly and to heat the entire compliment of workpieces in that zone as determined by the control setting.
Also of importance is the part that the flames play inside the furnace. The flames are developed in an area well over the top of the product. This may be done by the utilization of either long, short, or radiant burners. The burner choice depends on the furnace design. In any case, the flames are completely developed over the product in a combustion space. No combustibles are carried over into the next zone or into the exhaust flue.
The soak zone fires flames which develop an even heating head to the product. Subsequent zones then contain exhaust gases from the previous zones. Thoroughly fixed into the exhaust gases are diatomic gases. These absorb heat from the zone burners as they pass through a zone and they then rereadiate some of it to the steel. These exhaust gases are not only underneath the firing flames but are also cooler than the flames because of their closeness to the cold steel. This provides eveness of heat being transferred to the steel. In addition, the exhaust gases flow counter to the product flow. This counter flow provides convection heat from the gases to the colder steel.
In operation, the individual rows of steel product are individually subject to independent combinations of push and dwell periods as they progress through the zones and ultimately to the discharge. The periods of push and dwell are mutually independent of each other and are controlled by mill operations. The amount of push depends on the width of the product that must be discharged form the furnace. As described above, the workpieces progress through the zones, not in a slow continuous manner as in the continuous furnace, but in a halting manner. Thus, in an example of two rows of slabs, one row of slabs is pushed forward approximately four feet, causing the leading slab to be discharged. Even though the following colder pieces of steel enter the zones following a discharge, the firing rates continue almost unchanged because of the heat lag inherent in the steel and brickwork already in that zone and the resting time available to the colder piece.
It may readily be seen that the temperature control described above in any particular zone maintains primarily an average kind of temperature control supervision over the fuel firing in that zone, and cannot react nor accommodate localized cold areas in the zone, such as a skid mark.
Conventional heating practices commonly are unable to respond in full correspondence with the heating demand due to production requirements. Temperature uneveness or deficiency may result, particularly in the soak zone where it may not be possible to even out the steel temperature and erase the harmful skid marks. Also, when there is a delay in the furnace due to mechanical troubles, or mill delay, even though the firing rate in the heating zone may be cut back, there may be overheating and underheating of the steel because of excessive resting time of the steel on the water-cooled skids. Finally, when demand increases, the steel delivered onto the soaking hearth is generally heavily skid marked.